Nickel sulfamate baths are a common electrolyte formulation used in many advanced nickel plating applications such as various wafer level packaging (WLP) applications, and various engineering applications where low stress films are a requirement. These baths are typically composed of dissolved nickel sulfamate salts, boric acid, and in some formulations, small amounts of one or more plating additives for changing the deposit surface and stress properties (e.g. saccharin as a film stress reliever and brightener). In some systems, chloride ion is added to help aide and maintain proper dissolution at the nickel anode, particularly when sulfur containing nickel depolarized anode is not used. Typically, the target acidity of these baths is within a pH range broadly of about 3.0 to about 5.0, and sometimes within a more limited range of 3.5 to 4.5.
In a typical nickel electroplating process flow, such as that employed in a typical wafer level packaging (WLP) application, multiple semiconductor wafers are plated sequentially in each nickel sulfamate bath. Since deviations in bath composition can result in inferior electroplating, poor process performance, and potentially defects in the plated nickel layers, ideally, each wafer is plated under substantially the same process conditions, relatively invariant with time and constant over the plating of numerous wafers. In practice, however, maintaining constant process conditions in nickel sulfamate baths can pose a significant challenge.